Abstract
Connecting nanoscale mechanical resonators to microwave quantum circuits opens new avenues for storing, processing, and transmitting quantum information. In this work, we couple a phononic crystal cavity to a tunable superconducting quantum circuit. By fabricating a one-dimensional periodic pattern in a thin film of lithium niobate and introducing a defect in this artificial lattice, we localize a 6 gigahertz acoustic resonance to a wavelength-scale volume of less than one cubic micron. The strong piezoelectricity of lithium niobate efficiently couples the localized vibrations to the electric field of a widely tunable high-impedance Josephson junction array resonator. We measure a direct phonon-photon coupling rate $g/2\pi \approx 1.6 \, \mathrm{MHz}$ and a mechanical quality factor $Q_\mathrm{m} \approx 3 \times 10^4$ leading to a cooperativity $C\sim 4$ when the two modes are tuned into resonance. Our work has direct application to engineering hybrid quantum systems for microwave-to-optical conversion as well as emerging architectures for quantum information processing.
Highlights
Compact and low-loss acoustic wave devices that perform complex signal processing at radio frequencies are ubiquitous in classical communication systems [1]
In a series of remarkable experiments, thin-film [3], surface [4,5,6,7], and bulk acoustic wave resonators [8,9] made of piezoelectric materials have coupled gigahertz phonons with varying levels of confinement to superconducting circuits
At the heart of our device lies a suspended quasi-onedimensional phononic crystal fabricated from a 200-nmthick film of lithium niobate (LiNbO3)
Summary
Compact and low-loss acoustic wave devices that perform complex signal processing at radio frequencies are ubiquitous in classical communication systems [1] Much like their classical counterparts, emerging quantum machines operating at microwave frequencies [2] stand to benefit from their integration with these devices. The modes of phononic crystal cavities are confined to extremely small volumes (≲1 μm3) This leads to smaller forces for a given oscillating voltage when compared to approaches with transducer dimensions of tens to hundreds of microns [16]. It has only been possible to efficiently read out and couple to localized modes of phononic crystal cavities with optical photons where the electromagnetic energy is localized [10,17] We demonstrate the direct coupling of a superconducting circuit to a wavelength-scale phononic nanocavity, opening a new avenue in quantum acoustics
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